Fabrication of opto-electronic devices in GaAs requires the use of etch processes to produce features for both microoptical and microelectronic components. GaAs as used herein includes all compounds, crystalline and polycrystalline, containing gallium and arsenic with or without additional elements. Examples of features which are transferred into GaAs by etch processes include components such as waveguides, channels, facets, mesas and mirrors. To prepare components of this type having small, often sub-micron size dimensions, etch processes which provide a high degree of anisotropy are required. Wet etch techniques are, in general, unsuitable. Etch rates in wet processes are either isotropic or dependent on crystallographic orientation. Dry etch techniques, on the other hand, can avoid the dependence in rate (under optimum conditions) to crystallographic orientation and can provide the anisotropy required.
The majority of dry techniques used to provide an anisotropic etch in GaAs are ion-based processes which utilize chemistry to provide some form of reactive assistance. The most common techniques of this type include: Reactive Ion Etching (RIE), Reactive Ion Beam Etching (RIBE), and Ion Beam Assisted Etching (IBAE) [also known as Chemically-Assisted Ion Beam Etching (CAIBE)]. The chemistry utilized by these techniques for reactive assistance enhances etch rates, forms volatile etch products, and minimizes damage to the GaAs surface by energetic ions, neutrals, and/or radicals. In the dry etch processing of GaAs, ambients containing chlorine (atoms, molecules, neutrals, radicals) have been found most useful for providing suitable reactive assistance.
Some form of mask is required to protect desired regions of a substrate when using dry techniques to etch GaAs. Durable masks are often of particular value. The term "durable" as used herein defines the resistance of the mask to erosion during an etch process. An ideal mask is durable to the extent that it will not erode or change form during an etch process. For a mask to exhibit significant durability in the dry etch processing of GaAs, the ratio of the etch rate of the GaAs to that of the mask, i.e., the selectivity of the etch, must be high. Masks of materials of low durability are unsuitable for several reasons. First, the edge quality of etch features decreases as mask thickness increases. Second, mask features of dimensions smaller than the thickness of the mask are unstable and can break away or shift position during processing. Finally, mask erosion especially of edges can redeposit mask material into unwanted regions and degrade overall etch quality and uniformity.
In the dry etching of GaAs using chlorine for chemical assistance, few materials are known which when applied as thin layers (ca. 0.1 micron) offer the durability to survive an etch of from a few to many microns. Metals such as nickel (with titanium underlayer) and chromium and salts such as aluminum fluoride and strontium fluoride have been used with varying degrees of success. Wet techniques are frequently required to follow the dry etch processes to effect complete removal of these materials. What are needed are masks of high durability which are convenient to apply, pattern, and remove.